1. Field
One embodiment of the invention relates to, for example, a magnetic disk drive that refreshes data written to a disk, and also relates to a data refresh method applied to a magnetic disk drive.
2. Description of the Related Art
In recent years, the storage capacity of magnetic disk drives has increased. To increase the storage capacity, the recording density and track density of the magnetic disk have been raised. The higher the track density, the shorter the intervals of tracks (i.e., track pitch) on the disk will be. Each track has the same width as the head (more precisely, the write element included in the head). However, the width of distribution of write magnetic fields generated by the head during the data write is not always equal to the width of the head. The write magnetic fields are applied (leak to) to the periphery of the head. This phenomenon is called “write fringing.”
If the track pitch is decreased, the data (recorded data) in any track may be degraded when data is written to an adjacent track. This results from the write fringing and an error in positioning the head on the track (positioning error). If the recoded data (i.e., recorded signals) is degraded many times, it will become virtually unreadable. Even if error correction codes (ECC) are used in the most effective way possible, it is difficult to restore the data.
Therefore, in any recently developed magnetic disk drive, data refresh (rewriting) must be executed to restore the recorded data before the recorded data becomes unreadable through degradation. The data refresh is known as a process in which the degraded data is first read from a storage area of the track and then written back to the storage area, thus restoring the data in its normal state.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2004-273060 (the “prior art”) discloses the technique of refreshing the data written to a track adjacent to a track to which data has been written a specified number of times. In the prior art, the data recorded in a track adjacent (i.e., adjacent track) to a track to which data has been written the specified number of times is determined to be degraded. Then, the data recorded in the adjacent track is read from the adjacent track and temporarily stored in RAM, as data to be refreshed. The data so stored in RAM is written back to the storage area of the adjacent track. In other words, the data to be refreshed is read from the track and then written again to the track. This data rewriting, or data refresh operation, restores the data so that it is no longer degraded.
However, in the prior art, if the supply of power to the magnetic disk drive is interrupted during the data refresh operation, the data to be refreshed may be lost. To be more specific, the data may be lost if the supply of power is interrupted during the refresh-write operation, or while the data read from the track to be refreshed (hereinafter, referred to as “refresh-track”) is being written back to the refresh-track. The reason for this will be explained below.
Assume that the supply of power is interrupted during the refresh-write operation. Then, the data is incompletely written to the refresh-track. Consequently, the data in the refresh-track is effectively destroyed. At this point, the data that was read from the refresh-track is no longer in RAM. Even if the supply of power is restored, the data previously temporarily stored in RAM cannot be written to the refresh-track.
To prevent such a problem, a particular track on the disk may be used in place of RAM. That is, a particular track on the disk may be used as a backup-track in which the data in the refresh-track is temporarily saved (or backed up).
If a specified track on the disk is used as a backup-track, such data refresh as will be described will be executed in most cases. First, a backup-write operation is executed to write the data read from the refresh-track (hereinafter referred to as “original data”) to the backup-track provided on the disk. In the backup-write operation, the refresh-management information that represents the state of the backup-write operation is also written to the refresh-track.
When the backup-write operation is completed, a refresh-write operation is executed, reading the data written (or saved) to the backup-track and writing it back to the area of the refresh-track in which the data was initially recorded. When the refresh-write operation is completed, the refresh-management information written to the backup-track is rewritten to refresh-management information that comprises a completion flag. Thus, the completion flag is written to the backup-track. The completion flag indicates that the data (i.e., original data) has been refreshed. If the supply of power is interrupted during the refresh-write operation and the refresh-write operation is not completed, no completion flag will be written to the backup-track.
If the specified track (i.e., backup-track) provided on the disk is used in place of the above-mentioned RAM, the original data will not be lost even if the supply of power is interrupted during the refresh-write operation. This is because the original data is saved in the backup-track. Using the original data saved in the backup-track, the refresh-write operation not yet completed can be resumed. Whether the refresh-write operation has been completed or not can be determined in accordance with whether a completion flag has been written to the backup-track.
If the specified track (i.e., backup-track) provided on the disk is used in place of the above-mentioned RAM, however, a so-called “rotational delay” will be required. As will be described later in detail, the rotational delay occurs before the backup-write operation is started and before the refresh-write operation is started, and lasts until the head comes to the write-start position of the track. Once the rotational delay has occurred, the efficiency of the data refresh operation decreases.